Chloroplast Protein Synthesis in the Chromophytic Alga Olisthodiscus luteus: Cell Cycle Analysis

Inverted repeat of Olisthodiscus luteus chloroplast DNA contains genes for both subunits of ribulose-1,5-bisphosphate carboxylase and the 32,000-dalton Q(B) protein: Phylogenetic implications

The chloroplast DNA of the chromophytic alga Olisthodiscus luteus has been physically mapped with four restriction enzymes. An inverted repeat of 22 kilobase pairs is present in this 150-kilobase-pair plastid genome. The inverted repeat contains the genes for the large and small subunit polypeptides of ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and also codes for the 32,000-dalton Q(B) protein. These observations demonstrate that significant differences exist in chloroplast genome structure and organization among major plant taxa.

A conserved His-Asp signal response regulator-like gene in Heterosigma akashiwo chloroplasts

Regulation of gene expression in plastids may involve molecular components conserved from cyanobacteria-like ancestors. Among prokaryotes, genes are commonly regulated at the transcriptional level by 'two-component' or 'His-Asp' signal transducers, consisting of a 'sensor kinase', which autophosphorylates at a conserved histidine residue, and a cognate response regulator, which is phosphorylated by the sensor kinase at a conserved aspartate residue. A putative His-Asp response regulator gene (trg1: transcriptional regulatory gene 1) has been identified in the estuarine raphidophytic alga Heterosigma akashiwo. The chloroplast-encoded trg1 is 693 bp in length, contains no introns, and yields a conceptual translation product of 231 amino acids, with a predicted mass of 27 kDa. Homology searches suggest that Heterosigma trgl has an omnpR-like identity within the DNA-binding His-Asp family of response regulators. trg1 contains both the phosphorylation and DNA-binding domains which are present in prokaryote response regulators. Quantitative competitive RT-PCR showed that Heterosigma trg1 is expressed at low levels (5 microg per g total RNA). In contrast, psbA (a photosystem II component) transcript is abundant (60 mg per g total RNA). Cell cycle analysis showed that psbA abundance oscillates in response to light but trg1 mRNA levels are invariant. We hypothesize that a His-Asp phosphorelay mechanism may affect chloroplast genome transcription in a manner similar to bacterial signal transduction pathways in which 'sensor kinase' and cognate 'response regulator' proteins interact.

Purification and characterization of phosphoribulokinase from the marine chromophytic alga Heterosigma carterae

In this study we characterized phosphoribulokinase (PRK, EC 2.7.1. 19) from the eukaryotic marine chromophyte Heterosigma carterae. Serial column chromatography resulted in approximately 300-fold purification of the enzyme. A polypeptide of 53 kD was identified as PRK by sequencing the amino terminus of the protein. This protein represents one of the largest composite monomers identified to date for any PRK. The native holoenzyme demonstrated by flow performance liquid chromatography a molecular mass of 214 +/- 12.6 kD, suggesting a tetrameric structure for this catalyst. Because H. carterae PRK activity was insensitive to NADH but was stimulated by dithiothreitol, it appears that the enzyme may require a thioredoxin/ferredoxin rather than a metabolite mode of regulation. Kinetic analysis of this enzyme demonstrated Michaelis constant values of ribulose-5-phosphate (226 microM) and ATP (208 microM), respectively. In summary, H. carterae PRK is unique with respect to holoenzyme structure and function, and thus may represent an alternative evolutionary pathway in Calvin-cycle kinase development.

Photoregulation of Chloroplast Gene Transcription in the Chromophytic Alga Heterosigma carterae

Light acts as a complex regulator of cellular development and gene expression in photoautotrophs. Although light signals are highly effective in controlling cellular division and chloroplast biogenesis in the toxic marine alga Heterosigma carterae, their influence on gene expression has not been well characterized. To address this need cultures of H. carterae synchronized by an alternating light-dark regime were sampled through 12 h of light and 12 h of dark to characterize cell division, chloroplast complement, and chloroplast RNA abundance. These studies have identified a unimodal pattern of chloroplast transcriptional activity for a suite of cellular and photosynthetic genes. To determine the alga's response to a change in photoperiod, 12-h light/12-h dark-synchronized cultures were transferred to constant light and then periodically sampled. Although cellular and chloroplast division cycles remained synchronized in constant conditions for 24 h, the transcriptional apparatus responded by increasing ctRNA abundance within 45 min of the change in photoperiod. However, the ability of the alga to mount this rapid transcriptional response was limited to the first 2 h of the putative dark period. Thus, the chloroplast transcriptional apparatus of H. carterae may initiate a rapid, temporally gated response to a change in photoperiod that is independent of ongoing light-entrained cellular and chloroplast division cycles.

Chloroplast biosystematics: chloroplast DNA as a molecular probe

The classification of plants has traditionally been dependent upon the comparative analysis of morphological and biochemical data. In this paper the use of molecular probe analysis of chloroplast DNA (ctDNA) is used to expand the data base used in taxonomic studies. Chloroplast DNA size, homogeneity, the global arrangement of ctDNA structure, gene content, gene cluster array and gene sequence determination are discussed as useful criteria in the analysis of phylogenetic relationships.